Researchers have broken the code of an enzyme that plays a key role in the growth of most cancers, opening a path that could lead to a new class of anti-cancer drugs, according to a new study.

Other scientists who reviewed the study hail it as a breakthrough in fundamental cancer biology, but caution that much work remains before the exploit can be translated into next-generation therapies.

The enzyme, called telomerase, "is an ideal target for chemotherapy because it is active in almost all human cancer tumours, but inactive in most normal cells," says study leader Professor Emmanuel Skordalakes of Wistar Institute in Philadelphia.

"That means that a drug that deactivates telomerase would likely work against all cancers, with few side effects."

Cellular immortality

In humans, telomerase adds short sequences of DNA known as telomeres to the ends of chromosomes, thus preventing damage and the loss of genetic information when cells divide.

The enzyme is active mainly in cells that multiply frequently, such as embryonic stem cells, but is switched off in normal adult cells to avoid problems caused by runaway cell proliferation.

In cancer cells, however, telomerase is activated, allowing the disease cells to replicate endlessly and achieve what scientists call "cellular immortality," the hallmark of all cancers.

A decade-long search for telomerase inhibitors has, up to now, been hampered by a lack of knowledge of how the enzyme is structured.

Skordalakes and colleagues are the first to provide a complete view of a critically important protein within the telomerase molecule.

It reveals, at an atomic level, how the enzyme replicates the tips of chromosomes, a process critical to the development of tumours.

The breakdown of this same mechanism is also involved in the ageing process, which means any new inhibitor drugs might also help boost longevity.

Finding a source

The main obstacle blocking research on the complicated architecture of telomerase was simply gathering a sufficient quantity of the enzyme. Neither humans nor yeast, the standard laboratory sources, yielded enough.

Skordalakes screened a wide range of organisms, including protozoa and insects, before discovering that the red flour beetle (Tribolium castaneum) produced copious amounts of telomerase in a stable form.

"This was really the breakthrough," he says. "Once we found that the gene from this organism expressed the protein in the quantities we needed, we were able to move quickly."

The researchers used a technique called x-ray crystallography that analyses the patterns of x-rays beamed at molecule crystals to create a three-dimensional structure of the enzyme's active region.

Knowing how this region, called the telomerase reverse transcriptase protein (TERT), is structured made it possible to decipher how the enzyme works.

"For the first time, we can see how telomerase assembles at the end of chromosomes in initiate telomere replication," says Skordalakes.

More research

Several scientists who reviewed the study ahead of publication hailed its significance.

They added, though, that it was only one in a series of essential steps that led to any novel treatment for cancer.

"There is no doubt that having determined the structure of telomerase is extremely important in understanding cancer biology, and will probably help lead us in the development of telomerase inhibitors," says Professor Herbie Newell, head of Translational Research at Cancer Research UK.

"But whether these inhibitors will ultimately represent an effective class of anti-cancer treatments, it is too early to say."

Dr Jon Wilson, a scientist at The Institute of Cancer Research in London describes the research as a "major breakthrough" in the study of telomeres in the cancer process.

But he, too, cautioned that the development of drugs to deactivate the enzyme "in a therapeutic setting, although an exciting prospect, will require a great deal more research."